// Copyright 2011 Google Inc. All rights reserved.

package com.google.cloud;

import java.util.zip.Checksum;

/**
 * This class generates a CRC32C checksum, defined by rfc3720 section B.4.
 *
 *
 */
public final class Crc32c implements Checksum {

    private static final long[] CRC_TABLE = {
            0x00000000, 0xf26b8303, 0xe13b70f7, 0x1350f3f4,
            0xc79a971f, 0x35f1141c, 0x26a1e7e8, 0xd4ca64eb,
            0x8ad958cf, 0x78b2dbcc, 0x6be22838, 0x9989ab3b,
            0x4d43cfd0, 0xbf284cd3, 0xac78bf27, 0x5e133c24,
            0x105ec76f, 0xe235446c, 0xf165b798, 0x030e349b,
            0xd7c45070, 0x25afd373, 0x36ff2087, 0xc494a384,
            0x9a879fa0, 0x68ec1ca3, 0x7bbcef57, 0x89d76c54,
            0x5d1d08bf, 0xaf768bbc, 0xbc267848, 0x4e4dfb4b,
            0x20bd8ede, 0xd2d60ddd, 0xc186fe29, 0x33ed7d2a,
            0xe72719c1, 0x154c9ac2, 0x061c6936, 0xf477ea35,
            0xaa64d611, 0x580f5512, 0x4b5fa6e6, 0xb93425e5,
            0x6dfe410e, 0x9f95c20d, 0x8cc531f9, 0x7eaeb2fa,
            0x30e349b1, 0xc288cab2, 0xd1d83946, 0x23b3ba45,
            0xf779deae, 0x05125dad, 0x1642ae59, 0xe4292d5a,
            0xba3a117e, 0x4851927d, 0x5b016189, 0xa96ae28a,
            0x7da08661, 0x8fcb0562, 0x9c9bf696, 0x6ef07595,
            0x417b1dbc, 0xb3109ebf, 0xa0406d4b, 0x522bee48,
            0x86e18aa3, 0x748a09a0, 0x67dafa54, 0x95b17957,
            0xcba24573, 0x39c9c670, 0x2a993584, 0xd8f2b687,
            0x0c38d26c, 0xfe53516f, 0xed03a29b, 0x1f682198,
            0x5125dad3, 0xa34e59d0, 0xb01eaa24, 0x42752927,
            0x96bf4dcc, 0x64d4cecf, 0x77843d3b, 0x85efbe38,
            0xdbfc821c, 0x2997011f, 0x3ac7f2eb, 0xc8ac71e8,
            0x1c661503, 0xee0d9600, 0xfd5d65f4, 0x0f36e6f7,
            0x61c69362, 0x93ad1061, 0x80fde395, 0x72966096,
            0xa65c047d, 0x5437877e, 0x4767748a, 0xb50cf789,
            0xeb1fcbad, 0x197448ae, 0x0a24bb5a, 0xf84f3859,
            0x2c855cb2, 0xdeeedfb1, 0xcdbe2c45, 0x3fd5af46,
            0x7198540d, 0x83f3d70e, 0x90a324fa, 0x62c8a7f9,
            0xb602c312, 0x44694011, 0x5739b3e5, 0xa55230e6,
            0xfb410cc2, 0x092a8fc1, 0x1a7a7c35, 0xe811ff36,
            0x3cdb9bdd, 0xceb018de, 0xdde0eb2a, 0x2f8b6829,
            0x82f63b78, 0x709db87b, 0x63cd4b8f, 0x91a6c88c,
            0x456cac67, 0xb7072f64, 0xa457dc90, 0x563c5f93,
            0x082f63b7, 0xfa44e0b4, 0xe9141340, 0x1b7f9043,
            0xcfb5f4a8, 0x3dde77ab, 0x2e8e845f, 0xdce5075c,
            0x92a8fc17, 0x60c37f14, 0x73938ce0, 0x81f80fe3,
            0x55326b08, 0xa759e80b, 0xb4091bff, 0x466298fc,
            0x1871a4d8, 0xea1a27db, 0xf94ad42f, 0x0b21572c,
            0xdfeb33c7, 0x2d80b0c4, 0x3ed04330, 0xccbbc033,
            0xa24bb5a6, 0x502036a5, 0x4370c551, 0xb11b4652,
            0x65d122b9, 0x97baa1ba, 0x84ea524e, 0x7681d14d,
            0x2892ed69, 0xdaf96e6a, 0xc9a99d9e, 0x3bc21e9d,
            0xef087a76, 0x1d63f975, 0x0e330a81, 0xfc588982,
            0xb21572c9, 0x407ef1ca, 0x532e023e, 0xa145813d,
            0x758fe5d6, 0x87e466d5, 0x94b49521, 0x66df1622,
            0x38cc2a06, 0xcaa7a905, 0xd9f75af1, 0x2b9cd9f2,
            0xff56bd19, 0x0d3d3e1a, 0x1e6dcdee, 0xec064eed,
            0xc38d26c4, 0x31e6a5c7, 0x22b65633, 0xd0ddd530,
            0x0417b1db, 0xf67c32d8, 0xe52cc12c, 0x1747422f,
            0x49547e0b, 0xbb3ffd08, 0xa86f0efc, 0x5a048dff,
            0x8ecee914, 0x7ca56a17, 0x6ff599e3, 0x9d9e1ae0,
            0xd3d3e1ab, 0x21b862a8, 0x32e8915c, 0xc083125f,
            0x144976b4, 0xe622f5b7, 0xf5720643, 0x07198540,
            0x590ab964, 0xab613a67, 0xb831c993, 0x4a5a4a90,
            0x9e902e7b, 0x6cfbad78, 0x7fab5e8c, 0x8dc0dd8f,
            0xe330a81a, 0x115b2b19, 0x020bd8ed, 0xf0605bee,
            0x24aa3f05, 0xd6c1bc06, 0xc5914ff2, 0x37faccf1,
            0x69e9f0d5, 0x9b8273d6, 0x88d28022, 0x7ab90321,
            0xae7367ca, 0x5c18e4c9, 0x4f48173d, 0xbd23943e,
            0xf36e6f75, 0x0105ec76, 0x12551f82, 0xe03e9c81,
            0x34f4f86a, 0xc69f7b69, 0xd5cf889d, 0x27a40b9e,
            0x79b737ba, 0x8bdcb4b9, 0x988c474d, 0x6ae7c44e,
            0xbe2da0a5, 0x4c4623a6, 0x5f16d052, 0xad7d5351
    };

    private static final long LONG_MASK = 0xffffffffL;
    private static final long BYTE_MASK = 0xff;

    private long crc;

    public Crc32c() {
        crc = 0;
    }

    /**
     * Updates the checksum with a new byte.
     * @param b the new byte.
     */
    @Override
    public void update(int b) {
        long newCrc = crc;
        newCrc = updateByte((byte) b, newCrc);
        crc = newCrc;
    }

    /**
     * Updates the checksum with an array of bytes.
     * @param bArray the array of bytes.
     * @param off the offset into the array where the update should begin.
     * @param len the length of data to examine.
     */
    @Override
    public void update(byte[] bArray, int off, int len) {
        long newCrc = crc;
        for (int i = off; i < off + len; i++) {
            newCrc = updateByte(bArray[i], newCrc);
        }
        crc = newCrc;
    }

    public void update(byte[] bArray) {
        update(bArray, 0, bArray.length);
    }

    /**
     * Returns the value of the checksum.
     * @return the long representation of the checksum (high bits set to zero).
     */
    @Override
    public long getValue() {
        return crc;
    }

    /**
     * Returns the value of the checksum.
     * @return the 4-byte array representation of the checksum in network byte order (big endian).
     */
    public byte[] getValueAsBytes() {
        long value = crc;
        byte[] result = new byte[4];
        for (int i = 3; i >= 0; i--) {
            result[i] = (byte) (value & 0xffL);
            value >>= 8;
        }
        return result;
    }

    /**
     * Resets the crc.
     */
    @Override
    public void reset() {
        crc = 0;
    }

    private long updateByte(byte newByte, long crc) {
        byte b = (byte) (newByte & BYTE_MASK);
        int index = (int) ((crc ^ b) & BYTE_MASK);
        return (CRC_TABLE[index] ^ (crc >> 8)) & LONG_MASK;
    }
}